This book provides an overview of the newly emerged and highly interdisciplinary field of printed electronics* Provides an overview of the latest developments and research results in the field of printed electronics* Topics addressed include: organic printable electronic materials, inorganic printable electronic materials, printing processes and equipments for electronic manufacturing, printable transistors, printable photovoltaic devices, printable lighting and display, encapsulation and packaging of printed electronic devices, and applications of printed electronics* Discusses the principles of the above topics, with support of examples and graphic illustrations* Serves both as an advanced introductory to the topic and as an aid for professional development into the new field* Includes end of chapter references and links to further reading

1Introduction

Zheng Cui

1.1 What is Printed Electronics?

Printed electronics, as the name implies, is a type of electronics that are created by printing technology. To be more specific, it is an electronic science and technology based on conventional printing techniques as the means to manufacture electronics devices and systems. To most people, "printed electronics" is an unfamiliar phrase. Even experts in electronics may not have heard it. Many people may have it confused with conventional printing technology or mixed up with electronic printing. Conventional printing is for printing paper media, such as books, newspapers, and magazines. Even electronic printing is not printed electronics. Electronic printing is still conventional media printing but with more use of computers and electronic typesetting. A closer analogy to printed electronics would be electronics or integrated circuit (IC)-based electronics, rather than conventional printing. The aim of printed electronics is to make integrated electronic systems using printing technology instead of much more expensive and complex IC manufacturing technology.

Silicon-based IC technology has been in use for nearly 60 years. Modern silicon microelectronics and its manufacturing technology have evolved into an extremely complicated process. There are several hundreds of steps involved in producing a silicon IC chip, from the preparation of single crystal silicon substrates to making billions of transistors and getting these transistors interconnected, including repeated thin film deposition, lithography, etching, and packaging [1]. IC manufacturing has become so expensive that the latest deep UV photolithography system can cost tens of millions of dollars, whereas an extreme UV lithography system for making silicon chips at below 32 nm feature size has a price tag of more than $120 million [2]. The IC industry has become so investment intensive that only a handful companies in the world can afford to play in the field. On the other hand, printing is a very simple process compared to the IC manufacturing process, as illustrated in Figure 1.1 In order to turn a functional material into a functional structure or pattern on a silicon substrate, IC manufacturing has to go through thin film deposition, spin-coating photoresist layer, baking, photolithography, baking, developing, etching, and stripping of the photoresist masking layer. If printing is employed, the functional material can be directly printed as patterns onto the substrate. Only a subsequent annealing/sintering process is needed.

Printing is an additive manufacturing process, similar to the deposition process in micro- and nanofabrication [3] but combined with patterning. In printed electronics, the components of an electronic device or a system can be made by printing in additive fashion. For example, for a field-effect transistor, the source, drain, and gate electrodes, as well as semiconductor and insulating layers, can all be printed in ink forms and layer by layer onto a substrate. It is very much like color printing in a conventional printing press, where each color ink is printed sequentially and several color layers are overlaid to form the final color print. Because of its similarity to the printing process, the machine to print electronics is not much different from a conventional media printer. Figure 1.2 compares a conventional roll-to-roll paper media printer and an electronics printer. They look almost the same. The only difference is the inks they use. The inks for printing electronics have conducting, semiconducting, or dielectric properties. They are electronic materials, not pigment, which is the key for printing to be used for printed electronics.

Printed electronics originated from organic electronics. In 1977, Alan Heeger, together with Alan G. MacDiarmid and Hideki Shirakawa, discovered that polymer could be conductive by doping certain molecules [4], which earned them the Nobel Prize in Chemistry in 2000. This discovery completely overthrew the conventional wisdom that organic polymer materials are always insulators. Following the discovery of conductive polymers, organic semiconductor materials were developed in 1983 [5] and organic field-effect transistors (OFETs) were first made in 1986 [6]. In the same period, Dr. C.W. Tang at Kodak developed organic photovoltaic (OPV) materials [7] and later invented the organic light-emitting diode (OLED) [8], from which organic electronics as a field of scientific interest started.

The reason the scientific community got interested in organic electronics was not only due to scientific curiosity but more importantly that they foresaw the prospect of printing electronic devices from organic polymers that could be naturally made into ink forms. Once they could be printed, electronic devices could be made on a massive scale at low cost, very much like printing newspapers. So from the early stage of development, people made attempts to process organic electronic materials in solution forms to make transistors [9]. In 1994, a research group led by Francis Garnier first reported OFETs made on plastic substrates [10]. Although only electrodes were printed and the organic semiconductors were deposited by vacuum evaporation, the significance of the work was that it proved transistors could be made on plastic substrates, opening the era of plastic electronics. Fully printed transistors were reported in 1997 when Dr. Zhenan Bao, working at Bell Labs, printed all layers including conductor, semiconductor, and dielectrics onto polyester (PET) film by a screen printing technique [11]. More recently, Professor Sirringhaus at Cambridge University made fully printed organic transistors by the inject printing technique [12].

It is apparent that organic electronics had its eye on low cost printing electronics from the beginning of its development. However, for a very long period, printing did not become the mainstream fabrication means for making organic electronic devices. The main reason lies in the fact that the charge mobility, which is a key property of semiconductor material, for the solution form of organic semiconductors is always lower than those small molecular organic semiconductors that cannot be made into solution form and have to be deposited by vacuum evaporation. In other words, transistors made by printable organic materials are not as good as those made by vacuum evaporated organic materials.

Charge mobility is the speed of electronic charge (electrons for n-type semiconductor or holes for p-type semiconductor) movement in semiconductor materials. It determines how fast a transistor switches at an applied external electric field. Table 1.1 lists the charge mobility of commonly used inorganic semiconductor materials, in comparison with organic semiconductor. It shows that the charge mobility of organic semiconductor materials is far lower than inorganic semiconductors.

The research in organic electronics in its over 3 decades of development history has been mainly focused on how to improve the charge mobility, as it is obvious that only high mobility organic semiconductors have value in any practical applications. The last 25 years have indeed seen the steady improvement of charge mobility in organic semiconductors, as shown in Figure 1.3, which indicates the evolution of charge mobility from 1985 to 2010 for three different types of organic p-channel and n-channel semiconductor materials: vacuum-deposited small molecular organic materials, solution-processed small molecular organic materials, and solution-processed polymer materials [13].

(Adapted from [13] with permission from the Royal Society of Chemistry.)

The evolution curves reveal two things: first, there has been tremendous progress in improving the charge mobility of organic semiconductor materials. The mobility has increased 6 orders of magnitudes in the last 25 years. Second, the charge mobility of solution-processed polymeric organic semiconductor materials, though continuously improved, was always an order of magnitude lower than that of vacuum-deposited small molecular materials throughout the 25 years of development. Though the gap became smaller in the last few years due to the efforts in solution forms of organic semiconductors, small molecular organic semiconductor materials are still far better in terms of charge mobility [14]. As the ability to process organic semiconductor in solution form is the prerequisite of printing fabrication, low performance has prevented printing from becoming the preferred means of making organic...